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Jester009
Joined: 11 Sep 2007
Posts: 7
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 Posted: Wed Sep 12, 2007 9:14 pm |
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Thanks for the explanation. I have an oscilloscope. I will be able to acquire 8030 from www.futurlec.com. I don't think it will be available in the local market.
I found an already completed design here. Can you please tell me what you think about that...
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http://www.zendurl.com/j/jester9///6377051.pdf |
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SherpaDoug
Joined: 07 Sep 2003
Posts: 1640
Location: Cape Cod Mass USA
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| Posted: Thu Sep 13, 2007 7:18 am |
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| treitmey wrote: | If I was doing this porject I would buy something like this..
http://www.oselectronics.com/ose_p16.htm
Replace the pot that controls the voltage with a digital pot.
Then have the PIC adjust the digial pot,.. and it could also determine
when the relay tripped... and that corresponding voltage. and resulting current. V=IR |
From the looks of that device I don't think the knob connects to a pot. I think the knob turns a variac. The only way to control it with a PIC would be through some sort of motor.
_________________
The search for better is endless. Instead simply find very good and get the job done. |
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Humberto
Joined: 08 Sep 2003
Posts: 1215
Location: Buenos Aires, La Reina del Plata
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| Posted: Thu Sep 13, 2007 9:35 am |
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The original poster requested for a variable current generator to test relays.
Really I can´t understand what is the purpose to include a PIC in this project while
it is evident that he does not have enough experience to start such a complex project.
It would be very good to know:
1) It is mandatory to control it with a MCU?
2) Electrical characteristics of the relay to test.
3) Application/environment where it is going to be used.
4) Amount of relays to test.
5) Other.
Humberto |
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Jim Hearne
Joined: 22 Dec 2003
Posts: 109
Location: West Sussex, UK
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| Re: 4-20mA output |
| Posted: Thu Sep 13, 2007 9:48 am |
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| libor wrote: | (let me add a question to this already offtopic, but related hw thread, i did not dare to start one :-)
In a project I need to output 4-20mA. What do you suggest ? a PIC using a PWM output to control voltage on the current loop ? a separate DAC ic doing this ? or a digital potentiometer will do ?
Do I need a current measuring feedback going into az ADC pin to control the voltage output with the PIC ? ..or is there a simple open-loop voltage to current converter circuit with no feedback required ?
I would like to have a simple (too much precision is not required) solution. Is the 5V supply enough for the max 20mA output range? ...but how do I know what resistor value will be used in the receiving-end device, can I assume that max. 250 Ohms will be used, so the 5V supply is enough ?
Googling for solutions I have also found also some expensive SPI bus --> 4-20mA loop transducers ICs. ? should I go this way ? |
We use the Analog Devices AD421 for a 4-20ma output on many of our devices and it works very well. It's easy to drive with a SPI interface.
The only thing is it can be a bit expensive. |
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Jester009
Joined: 11 Sep 2007
Posts: 7
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| Posted: Thu Sep 13, 2007 9:50 am |
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| Humberto wrote: | The original poster requested for a variable current generator to test relays.
Really I can´t understand what is the purpose to include a PIC in this project while
it is evident that he does not have enough experience to start such a complex project.
It would be very good to know:
1) It is mandatory to control it with a MCU?
2) Electrical characteristics of the relay to test.
3) Application/environment where it is going to be used.
4) Amount of relays to test.
5) Other.
Humberto |
Hi Humberto,
Thanks for replying. I must admit that I have very little knowledge when it comes to electronics. 
1). The user must be able to select the current through an interface in the computer. So I think it's mandatory to use a MCU.
2). What I have to test is a numerical overcurrent relay. Details are as follows.
| Code: | Manufacture : ALSTHOM
Model: KCGG140
Nominal current 5A (CT secondary rating)
Auxiliary supply 24-125 DC |
I have to do a demonstration only on that particular relay. Making it to suit other kinds of relays will be included in further improvements. Right now I'm trying come up with an unit to test only that particular relay... |
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Ttelmah
Guest
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| Posted: Thu Sep 13, 2007 10:26 am |
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Right at the start, you were talking about AC. Now most relays of this sort, will operate on AC, or DC. If this unit is like this, and you don't _have_ to have AC, then you can save a huge amount of work, by just using a PWM DC drive. If you require to test 'both ways', to ensure that the unit does support AC, then use an H-Bridge.
Controlling DC, is a lot easier...
Best Wishes |
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Humberto
Joined: 08 Sep 2003
Posts: 1215
Location: Buenos Aires, La Reina del Plata
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| Posted: Thu Sep 13, 2007 1:16 pm |
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| Quote: |
I must admit that I have very little knowledge when it comes to electronics
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Don´t worry, we all had had very little knowledge in the beginnings...
The ¨relay¨ you are talking it is not a simple one (coil + contacts) as we supposed. 
The ALSTHOM KCGG140 is a MCU based relay with fully built-in integrated protection,
control and monitoring, that is, a sofisticated module plenty of electronics inside.
It include an embedded communication interface that enable you to connect it
to a PC or a SCADA link, hence you have 95% of your task already done.
Humberto
When everything else fails and nothing is running at all, read the instructions. |
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